1. Field of the Invention
The present invention relates to a drum type image scanner, and especially to a drum type image scanner having an optical unit movable along a straight track rail.
2. Description of the Related Art
A drum type image scanner generally includes a freely rotatable cylinder, or a drum, on which a sheet of an original is mounted. FIG. 1 is a schematic view illustrating a transparent cylinder 100 applied to a drum type image scanner.
The cylinder 100 is provided with a fixture disc 102 on one end thereof. The fixture disc 102 has a cylinder support shaft 104 projecting at its opposite side to the cylinder 100, and a tapered shaft 106 at the end of the support shaft 104. The cylinder 100 and the fixture disc 102 are integrally manufactured so that the axis of the cylinder 100 and those of the cylinder support shaft 104 and the tapered shaft 106 coincide with each other.
The tapered shaft 106 is inserted into a chuck 110 connected to a cylinder drive motor and fixed with a clamp ring 112 of the chuck 110. The cylinder 100 is thus rotatably held on the drum type image scanner.
The drum type image scanner further includes a straight track rail, a track table movable along the track rail, and an optical unit fixed on the track table. The optical unit includes a light source and an optical sensor.
The drum type image scanner functions in the following manner. An original held on the circumference of the cylinder 100 is irradiated with light emitted from the light source while the cylinder 100 is rotated and the optical unit is moving along the track rail in a direction parallel to the axis of the cylinder 100 (that is, in the subscanning direction). The photosensor receives light transmitted through or reflected from the original.
In such a conventional image scanner, however, the distance between the surface of the cylinder 100 and the photosensor is not always constant throughout scanning, which causes an out-of-focus or distorted image and poor resolution. The distance change is attributable to lack of parallelism of the axis of the cylinder 100 to the straight track rail, or to mismatch between the physical central axis of the cylinder 100 and its rotational axis. The distance change is seen as circumferential deviation of the cylinder 100.
The circumferential deviation of the cylinder 100 can be ascribed to mismatch between the central axis of the cylinder 100 and the rotational axis of the cylinder drive motor. The mismatch sometimes results from foreign matters such as dust between the tapered shaft 106 and the chuck 110.
Special care is thus required to check the outer face of the tapered shaft 106 and the inner face of the chuck 110 for foreign matters when the cylinder 100 is attached to the shaft of the cylinder drive motor. After the cylinder is attached, the circumferential deviation of the cylinder 100 is checked at regular intervals during test operation so as to ensure that the distance between the surface of the cylinder 100 and the photosensor is kept constant during scanning. When undesirable deviation of the cylinder 100 is detected, the cylinder 100 is reattached to the track rail and adjusted to make the circumferential deviation within an allowable range, thereby ensuring that the axis of the cylinder 100 is parallel to the track rail. These adjustments requires time, labor, and skills.